These CAIs are really solid products and make both an audible, performance and fuel economy difference. In my opinion, absolutely worth the investment. For me, I wanted to approach the reviews from a "butt-dyno" point of view, and also try to measure the audible changes. I took sound spectrum samples with both the OEM setup, and then the S1/S2 kits at the engine, cabin, and exhaust. You can definitely see the results of the change, which validates what you will hear.

These are set for the M3, but I could potentially do more guides for other models if there is a need.. I have access to the following:

2005 X3 E83
2007 550 E60
2009 335i E92
2011 M3 E92

I would love to do more of these guides, and if there is enough interest in other models, I will speak with aFe about it and see if they would like to partner again for the work. If you would like a guide for one of those models, PLEASE say so!

So fantastic question honestly, between the S1 and S2. Out of the box, the S1 has *slightly* less performance because it uses the existing OEM intake tube. That said, I was completely confused as well looking between the two products.

Really? I think it comes down to appearance primarily. The S1 kit has all of that amazing billeted aluminum, in both the cover and the intake, while the S2 has not. Considering there is a fairly substantial design difference between the two kits, it may be a manufacturing change as well.

Now onto *pure* supposition to be clear: Considering the S1 has that metal lid, I suspect the S2 has slightly more volume or space in the airbox area as it seals up against the hood instead. I am guessing the S1 was designed first, and then aFe found both a more efficient way of manufacturing (hence cheaper) and better performing S2 design. That is purely a guess though. I expected the heat shield and filter to be the same between the two kits, and they are not.. different in design, shape and part number. No clue of my guess is right though.

aFe has been really wonderful to work with ever since I approached them with my proposal for partnership, instead of pure sponsorship per se. I wanted to *earn* the work with and from them. I have an S2 kit in my E46 M3 now because of it.. *Grin* They are fantastic, and I really hope to do more work with them. I believe I could do an install guide for the exhaust of equal quality, and would do CAI DIYs for the other models I have access too as well.

I didn't make any sound recordings though perhaps I should have.. being *really* in tune with my car, I could hear the difference and wanted to provide some measured and demonstrable difference if I could.